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Friday, October 7, 2011

Faster than light

Last week CERN announced that they had observed neutrinos to be traveling faster than the speed of light in a vacuum. I seem to recall Fermilabs finding similar (although inconclusive) results a couple years ago.

I realize it may be asking too much of particle physicists to stoop so low as to ask for help from an astronomer. But it turns out that the astronomy community has been aware for quite some time that when a star goes supernova, the neutron stream from that reaches earth some significant amount of time (minutes or hours, not nanoseconds) earlier than the light from the same event does. It has been previously assumed that the neutrinos were somehow being released earlier than the light, which may still be true. But if instead the time lag is due wholly or partly to neutrinos traveling faster than the light, then the further away the supernova is, the greater the timelag should be. That should be a pretty easy lookup for somebody. I may look it up myself this weekend, but I'm at work and my lunch break is rather short.

Here's the CERN press release, for those who have only heard this from Fox News:

OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso
Geneva, 23 September 2011. The OPERA1 experiment, which observes a neutrino beam from CERN2 730 km away at Italy’s INFN Gran Sasso Laboratory, will present new results in a seminar at CERN this afternoon at 16:00 CEST. The seminar will be webcast at Journalists wishing to ask questions may do so via twitter using the hash tag #nuquestions, or via the usual CERN press office channels.

The OPERA result is based on the observation of over 15000 neutrino events measured at Gran Sasso, and appears to indicate that the neutrinos travel at a velocity 20 parts per million above the speed of light, nature’s cosmic speed limit. Given the potential far-reaching consequences of such a result, independent measurements are needed before the effect can either be refuted or firmly established. This is why the OPERA collaboration has decided to open the result to broader scrutiny. The collaboration’s result is available on the preprint server

The OPERA measurement is at odds with well-established laws of nature, though science frequently progresses by overthrowing the established paradigms. For this reason, many searches have been made for deviations from Einstein’s theory of relativity, so far not finding any such evidence. The strong constraints arising from these observations makes an interpretation of the OPERA measurement in terms of modification of Einstein’s theory unlikely, and give further strong reason to seek new independent measurements.

“This result comes as a complete surprise,” said OPERA spokesperson, Antonio Ereditato of the University of Bern. “After many months of studies and cross checks we have not found any instrumental effect that could explain the result of the measurement. While OPERA researchers will continue their studies, we are also looking forward to independent measurements to fully assess the nature of this observation.”

“When an experiment finds an apparently unbelievable result and can find no artefact of the measurement to account for it, it’s normal procedure to invite broader scrutiny, and this is exactly what the OPERA collaboration is doing, it’s good scientific practice,” said CERN Research Director Sergio Bertolucci. “If this measurement is confirmed, it might change our view of physics, but we need to be sure that there are no other, more mundane, explanations. That will require independent measurements.”

In order to perform this study, the OPERA Collaboration teamed up with experts in metrology from CERN and other institutions to perform a series of high precision measurements of the distance between the source and the detector, and of the neutrinos’ time of flight. The distance between the origin of the neutrino beam and OPERA was measured with an uncertainty of 20 cm over the 730 km travel path. The neutrinos’ time of flight was determined with an accuracy of less than 10 nanoseconds by using sophisticated instruments including advanced GPS systems and atomic clocks. The time response of all elements of the CNGS beam line and of the OPERA detector has also been measured with great precision.

"We have established synchronization between CERN and Gran Sasso that gives us nanosecond accuracy, and we’ve measured the distance between the two sites to 20 centimetres,” said Dario Autiero, the CNRS researcher who will give this afternoon’s seminar. “Although our measurements have low systematic uncertainty and high statistical accuracy, and we place great confidence in our results, we’re looking forward to comparing them with those from other experiments."

“The potential impact on science is too large to draw immediate conclusions or attempt physics interpretations. My first reaction is that the neutrino is still surprising us with its mysteries.” said Ereditato. “Today’s seminar is intended to invite scrutiny from the broader particle physics community.”

The OPERA experiment was inaugurated in 2006, with the main goal of studying the rare transformation (oscillation) of muon neutrinos into tau neutrinos. One first such event was observed in 2010, proving the unique ability of the experiment in the detection of the elusive signal of tau neutrinos.


  1. Light slows down in a medium. Is it possible that light from supernovas slows down in the not-quite-vacuum of interstellar space more than neutrinos do?

  2. Early expansionary period withstanding, the observable speed of light in interstellar space appears to be a rock solid constant. 186,000 miles per second; it's not just a good idea, it's the law. If it did prove to be less, that would have greater ramifications for our current understanding of physics and the Standard Model than even the possibility that neutrinos violate the light-limit.

  3. The observable speed of light in interstellar space is not constant, it varies by wavelength. This has been observed with pulsars, where high frequency radio emissions are observed before lower frequencies.

    This is because the interstellar medium is not a perfect vacuum: it has a refractive index, so light is slowed down a very tiny bit. And this varies by wavelength like optical dispersion.

  4. Yes, and light is "slowed" even more (substantially more) by refraction such as gravitational lensing. But even on the scale of intergalactic space, this is infinitesimal. What CERN is observing is not. We're talking about a difference of 18 meters after traveling only 720 kilometers. That's huge.

    Now, at that scale, if the CERN numbers are correct light should have reached us three YEARS before we visually observed the SN1987A supernova in the Large Magellanic Cloud. In fact, we observed the neutron stream only three hours prior.

  5. Correlation does not prove causation. It is assumed that the neutron pulse which occurred three hours before sn1987a was observed emanated from sn1987a. But three years before that, there were no neutron telescopes. It is possible, however unlikely, that the neutron pulse observed that day was in no way related to that particular supernova.

  6. @ Paul

    That is a very interesting point. I haven't had time to look at other data from other supernovae, but it seems like the number of instances we've observed the same supernova both visually and by neutron stream is quite a lot smaller than I assumed.

    Your responses to this and also my post about the Tiger Stripes has caused me to reflect quite a lot about the nature and application of Occam's Razor in astronomy particularly and science generally.

    In this case, which is the "simpler" explanation; that these two very rare occurrences happened within three hours of each other, or that the Standard Model is fundamentally flawed? Or, as some people have actually proposed, did the physicists at CERN actually forget to compensate for the fact that the earth is curved?

    That last one is a joke, by the way, or at least theindividuals who proposed it were. The track is of course equally curved for light as well as neutrons.

  7. Reading back over this comment thread, a point of clarification is in order.

    The speed of light, emphatically, is NOT dependent of frequency. Lower frequency electromagnetic waves are refracted more than high frequency electromagnetic waves. Thus, if LF waves happen to be refracted, then they have to travel farther than HF waves propagated from the same source. Because they travel farther at exactly the same speed, they arrive at the same "destination" later than their HF counterparts.